This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The E2A proteins E12 and E47 are expressed throughout B-cell development to activate B-cell specific gene expression and immunoglobulin gene rearrangement. Here we propose to examine the cellular localization of E12 and E47 during distinct stages of B-cell development utilizing biarsenical-tetracysteine labeling. E12 and E47 are transcriptional regulators that control developmental progression, cellular expansion, survival and gene rearrangement in developing lymphoid cells. How E12 and E47 regulate this diverse set of biological activities is unknown. Additionally it is unclear where E12 and E47 are localized during the distinct stages of lymphocyte development. In this application we propose to tag E12 and E47 utilizing the biarsenical-tetracysteine epitope. We would examine localization by fluorescence as well as EM studies. Ultimately we would like to combine electron microscopy and 3D-FISH to examine the localization of these transcription factors in the immunoglobulin locus. Data obtains from the studies may obtain insights into the mono-allelic activation of immunoglobulin gene rearrangement.